Suppose you have locked a diagram, and then you move the class around on the diagram followed by changing the name of the class.

Making an edit can affect more than the resource that you locked (in this case, the class is also modified).  So, locking a resource in Design Manager (in "Actively Managed" mode) followed by working offline with the resources that you locked is not supported.  If this was to be supported, you would need to know which resources you are about to modify in advance, which can be complicated!

If, however, you wish to lock a lot of resources -- perhaps even an entire model -- and you want to work offline, you can try the "Managed Externally by Import Definition" mode, which allows you to completely work offline, then import the model you have edited onto the Design Management server.